Efficient access to storage devices with usage bitmaps

ABSTRACT

Upon receiving a request to allocate a storage region, a storage device may initialize the contents of the storage device to default values (e.g., zero) in order to avoid problems arising from unknown data stored in the locations of the storage region (e.g., upon writing a data set to a location involved in a mirroring relationship, uninitialized data in the corresponding mirror location may result in a mismatch that jeopardizes the written data). However, initializing the storage device may be time-consuming and inefficient. Instead, a usage bitmap may be generated that, for respective location sets of the storage region, indicates whether values exist in the location. A read request may be fulfilled by examining the usage bitmap to determine whether values exist in the specified location, and if not, the default value may be returned without accessing the storage device. Other efficiencies may also be achieved using the usage bitmap.

BACKGROUND

Within the field, many scenarios involve the storage of values to astorage device set comprising at least one storage device, such as acollection of hard disk drives or a Redundant Array of Inexpensive Disks(RAID). When a storage device is first provided, or when capacity on astorage device is deallocated and then reallocated, the physical mediaof the storage device may be initialized; e.g., the locations of thestorage device may be wiped to a default value. This initialization mayenable a process that accesses a location of the storage device where avalue not yet been written to receive a specific value rather than anarbitrary value, including the previously written value in a deallocatedarea.

The initialization of a storage device may also be significant instorage scenarios involving a location set of comprising at least twoassociated locations, where the values stored therein have a particularrelationship. As a first example, in a mirroring resiliency plan,identical copies of a storage set may be stored at two or more locations(often on different storage devices), such that a value stored in onelocation is anticipated to match the value stored in one or more otherlocations involved in the mirroring relationship. As a second example,in a parity resiliency plan, when a set of data values is written todata locations of a location set, a parity of the written values may becalculated and stored in a parity location of the location set that isreserved for the parity value, which may enable error detection, errorcorrection, and the reconstruction of the data values in the event of afailure of some of the storage devices storing the values of thelocation set. In these scenarios, a failure to initialize a location mayimpact the confidence in the validity of the associated data sets; e.g.,if data is written to a first mirror location but a failure occursbefore the same data can be written to the associated location, amismatch may arise that erodes confidence in the value stored in thefirst mirror location, and possibly in the remainder of the storagedevice set.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key factors oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

While initializing the values of a storage device may avoid somedisadvantages, the process of initialization may present several formsof inefficiency. As a first example, a request to allocate capacity in astorage region may involve a write of every location of aninitialization value (e.g., zero) to the entire contents of the storageregion, which may comprise a protracted process, particularly forstorage regions of large capacity and/or slow throughput. As a secondexample, the initialization of a location may be followed by a firstaccess that comprises a write request; the initialization in this casetherefore did not achieve any value, and may have been avoidable. As athird example, initializing a storage region with a default value andthen reading unwritten locations of the storage region may predictablyreturn the default value, and may have been avoided if the location mayhave been determined to be unwritten with a value.

Presented herein are techniques for reducing the inefficiencies causedby the initialization of a storage region by simply not initializing thestorage region (e.g., not writing the default value to the locations ofthe storage region). Instead, a computer or other device may generate ausage bitmap that records, for respective location sets of the storageregion, a bit indicating whether or not the location set has beenwritten since the storage region was allocated. Upon receiving a readrequest specifying a location, the computer may check the usage bitmapto determine whether data values have been written to the location setcomprising the location. If the usage bitmap indicates that values existin the locations of the location set, the read request may be satisfiedby reading and returning the value stored in the location on the storagedevice. However, if the usage bitmap indicates that values do not existin the locations of the location set, the computer may refrain fromaccessing the storage device, and may simply return the default value.These and other uses of a usage bitmap generated as presented herein mayimprove the efficiency of the storage set.

To the accomplishment of the foregoing and related ends, the followingdescription and annexed drawings set forth certain illustrative aspectsand implementations. These are indicative of but a few of the variousways in which one or more aspects may be employed. Other aspects,advantages, and novel features of the disclosure will become apparentfrom the following detailed description when considered in conjunctionwith the annexed drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating an exemplary method of readingvalues from a storage set.

FIG. 2 is an illustration of an exemplary computer-readable mediumcomprising processor-executable instructions configured to embody one ormore of the provisions set forth herein.

FIG. 3 illustrates an exemplary computing environment wherein one ormore of the provisions set forth herein may be implemented.

DETAILED DESCRIPTION

The claimed subject matter is now described with reference to thedrawings, wherein like reference numerals are used to refer to likeelements throughout. In the following description, for purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of the claimed subject matter. It may beevident, however, that the claimed subject matter may be practicedwithout these specific details. In other instances, structures anddevices are shown in block diagram form in order to facilitatedescribing the claimed subject matter.

A. Background

Within the field of computing, many scenarios involve the accessing ofnonvolatile storage devices of a storage device set. Such storagedevices may comprise many types (e.g., hard disk drives; solid-statestorage devices; and magnetic or optical discs), and may be utilized tostore many types of data (e.g., files, email messages, database records,or media objects, or combinations thereof). The storage devices may beattached to a particular computer or device, or may be connected to andaccessed through a network. The storage devices may also operateindependently (e.g., without intercommunication or even mutual awarenessof other storage devices of the storage device set); with loosecollaboration (e.g., in communication and exchanging notifications ofstatus); or with tight interoperation (e.g., a set of hard disk drivesloaded into a storage controller, such as a Redundant Array ofInexpensive Disks (RAID) appliance). Also, the interoperation and/or useof the storage devices may result in logical or practical partitionsbetween the data stored thereamong, or may result in an aggregation ofdata. For example, the available capacity of two or more storage devicesmay be aggregated into a storage pool that is presented to a computer asa single integrated volume.

The storage devices may also implement various layout plans, such asspanning (e.g., logically concatenating the capacity of the storagedevices); striping (e.g., interleaving sequential logical addressesacross the storage devices); mirroring (e.g., storing identical copiesof a data set on respective storage devices of the storage device set,and maintaining the identity of the copies by automatically andconcurrently applying changes to the data set to all of the copies); andparity calculations (e.g., when data is stored on one or more storagedevices, a checksum of the stored data may automatically be calculatedand stored on a different storage device, e.g., in order to enablerecovery from a failure of a storage device). Additionally, the capacityof the storage devices may be provisioned according to variousprovisioning plans. As a first example, in a fixed provisioning plan,when a request to allocate a storage region is received, one or morestorage devices may promptly allocate all of the capacity requested inthe provisioning request, and may inform the requesting user or processthat such capacity is available (and may optionally initialize thecapacity with a default value). As a second example, in a delayedprovisioning plan, when a request to allocate a storage region isreceived, the storage devices may indicate that the storage region isavailable for writing, but may not promptly allocate capacity for thestorage region. Rather, the storage devices may await an access requestto a location within the storage region, and upon receiving the accessrequest, may promptly allocate physical storage (e.g., an extent) of astorage device and bind it to the logical address of the location. This“just-in-time” delayed provisioning may present some performanceimprovements (e.g., promptly fulfilling a request to generate thestorage device, and/or a more efficient allocation of the availablecapacity of the storage device set than allocating capacity for astorage region that remains unused). As an additional variation, in a“thin” provisioning plan, the storage device set may authorize requeststo allocate capacity that exceed the available capacity of the storagedevice set. Instead, the storage device set may allocate the capacity asused, and upon running low on available capacity, may notify anadministrator to add storage devices that provide additional capacity.These and other types of layout and/or provisioning plans may be appliedto the storage devices.

Upon allocating a storage region of a storage device, the storage devicemay be configured to initialize the locations of the storage region withdefault values (e.g., zero) in order to remove the uninitialized valuesfrom the storage region, such as the contents of a previouslydeallocated storage region. The initialization may be significant forseveral reasons. As a first example, a process may request a read from alocation of the storage region before writing a value to it. Withoutinitialization, the value stored at the location may be unpredictable,and the behavior of the process after reading the unpredictable valuemay be non-deterministic or unexpected. As a second example, somestorage regions may store location sets respectively comprising two ormore locations having an association. The relationship may becompromised if a value is stored at a first location of the location setwhile arbitrary data is stored in the associated locations of thelocation set. As a first example, mirrored storage sets may involve adata set that is stored as two or more identical copies in differentlocations, and that are kept in synchrony by applying a write to onelocation to the corresponding mirrored locations. If a value is writtento a first such location, but the mirrored locations are notinitialized, a mismatch may be detected that jeopardizes the confidencethat the value written to the first location is valid. As a secondexample, a location set may involve two or more data values that, whenstored to the location set, result in the calculation of a parity valuefor the data values that is stored in a parity location of the locationset. The parity value may enable the detection and/or correction oferrors. The parity values may also enable a reconstruction of data ofthe location set that was stored on a storage device that has failed;e.g., the values stored in the other locations of the location set,along with the parity value, may be used to infer the value stored onthe failed storage device and the replacement of the value in asubstitute storage region. However, the presence of an uninitializedvalue in any of the locations of the location set may result in thereconstruction of incorrect data into the substitute storage region. Theinitialization of the values in a newly allocated storage device mayavoid these and other problems in various computing scenarios.

However, the initialization of values of a storage region may alsopresent several forms of inefficiency. As a first example, the processof writing a default value to an entire storage set, in response to arequest to allocate a storage region, may take a long time, and maydelay the fulfillment of the allocation request. As a second example, inmany cases, the writing of the default value to a location may providelittle or no value. As a first such example, if the initialization of alocation is followed by a write of a data value to the location (withoutfirst reading the value stored there), the initialization of thelocation with a default value may have provided no value. As a secondsuch example, if the initialization of a location is following by a readof the data value at the location (without first writing a value to thelocation), the result may be predicted as the default value withoutreading the value from the location of the storage device. For these andother reasons, the initialization of the locations of the storage regionmay unhelpfully consume the time and resources of the computer andstorage devices without providing value or advantage.

B. Presented Techniques

Presented herein are techniques for presenting values stored in thelocations of a storage region of a storage device in a comparativelyperformant manner. In accordance with these techniques, a newlyallocated storage set may not be initialized with the default values.Rather, a usage bitmap may be generated, where respective bits indicate,for respective location sets of the storage region, whether values havebeen stored and exist in the location set. Read requests may thereforebe satisfied in part with reference to the usage bitmap; e.g., the

C. Exemplary Embodiments

FIG. 1 presents a first embodiment of these techniques, illustrated asan exemplary method 100 of presenting values stored in location setscomprising at least two locations of a storage set of a storage regioncomprising a usage bitmap. The exemplary method may be implemented,e.g., as a set of instructions stored in a memory component of acomputer (e.g., a memory circuit, a platter of a hard disk drive, asolid-state storage device, or a magnetic or optical disc) that, whenexecuted by a processor of the computer, cause the computer to presentthe values stored in the location sets according to the techniquespresented herein. The exemplary method 100 begins at 102 and involvesexecuting 104 on the processor instructions configured to cause thecomputer to operate according to the techniques presented herein. Inparticular, the instructions are configured to, upon receiving a readrequest for a value at a location, determine whether the usage bitmapindicates that value exist for the location set comprising the location.If the usage bitmap indicates that values exist for the location setcomprising the location, the instructions are configured to return 106the value stored at the location. However, if the usage bitmap indicatesthat values do not exist for the location set comprising the location,the instructions are configured to return 110 a default value. In thismanner, the exemplary scenario achieves the presentation of values ofthe storage set in accordance with the techniques presented herein, andso ends at 112.

Still another embodiment involves a computer-readable medium comprisingprocessor-executable instructions configured to apply the techniquespresented herein. Such computer-readable media may include, e.g.,computer-readable storage media involving a tangible device, such as amemory semiconductor (e.g., a semiconductor utilizing static randomaccess memory (SRAM), dynamic random access memory (DRAM), and/orsynchronous dynamic random access memory (SDRAM) technologies), aplatter of a hard disk drive, a flash memory device, or a magnetic oroptical disc (such as a CD-R, DVD-R, or floppy disc), encoding a set ofcomputer-readable instructions that, when executed by a processor of adevice, cause the device to implement the techniques presented herein.Such computer-readable media may also include (as a class oftechnologies that are distinct from computer-readable storage media)various types of communications media, such as a signal that may bepropagated through various physical phenomena (e.g., an electromagneticsignal, a sound wave signal, or an optical signal) and in various wiredscenarios (e.g., via an Ethernet or fiber optic cable) and/or wirelessscenarios (e.g., a wireless local area network (WLAN) such as WiFi, apersonal area network (PAN) such as Bluetooth, or a cellular or radionetwork), and which encodes a set of computer-readable instructionsthat, when executed by a processor of a device, cause the device toimplement the techniques presented herein.

An exemplary computer-readable medium that may be devised in these waysis illustrated in FIG. 2, wherein the implementation 200 comprises acomputer-readable medium 202 (e.g., a CD-R, DVD-R, or a platter of ahard disk drive), on which is encoded computer-readable data 204. Thiscomputer-readable data 204 in turn comprises a set of computerinstructions 206 configured to operate according to the principles setforth herein. In one such embodiment, the processor-executableinstructions 206 may be configured to perform a method of reading valuesfrom a storage set, such as the exemplary method 100 of FIG. 1. Someembodiments of this computer-readable medium may comprise anontransitory computer-readable storage medium (e.g., a hard disk drive,an optical disc, or a flash memory device) that is configured to storeprocessor-executable instructions configured in this manner. Many suchcomputer-readable media may be devised by those of ordinary skill in theart that are configured to operate in accordance with the techniquespresented herein.

D. Variations

The techniques discussed herein may be devised with variations in manyaspects, and some variations may present additional advantages and/orreduce disadvantages with respect to other variations of these and othertechniques. Moreover, some variations may be implemented in combination,and some combinations may feature additional advantages and/or reduceddisadvantages through synergistic cooperation. The variations may beincorporated in various embodiments to confer individual and/orsynergistic advantages upon such embodiments.

D1. Scenarios

A first aspect that may vary among embodiments of these techniquesrelates to the scenarios wherein such techniques may be utilized. As afirst variation of this first aspect, these techniques may be used withmany types of storage devices, including hard disk drives, solid-statestorage devices, nonvolatile memory circuits, tape-based storagedevices, and magnetic and optical discs. Such storage devices may alsobe directly connected to a device (such as a computer) implementingthese techniques; may be accessible through a wired or wireless localarea network (e.g., an 802.11 WiFi network or ad-hoc connection, or aninfrared connection); and/or may be accessible through a wired orwireless wide-area network (e.g., a cellular network or the internet).Moreover, these techniques may be used with two or more storage devicesoperating independently (e.g., storage devices that are accessedindependently through a software process); operating with looseinteroperation (e.g., storage devices that operate independently butthat are informed of and may communicate with the other storage devicessharing the storage set); or operating with tight interoperation (e.g.,a Redundant Array of Inexpensive Disks (RAID) controller managingseveral storage devices as components of a storage system).

As a second variation of this first aspect, these techniques may be usedin conjunction with many types of storage sets comprising various typesof data sets, including binary storage systems storing various types ofbinary objects; file systems storing files; media libraries storingmedia objects; object systems storing many types of objects; databasesstoring records; and email systems storing email messages.

As a third variation of this first aspect, portions or all of thesetechniques may be implemented within one or more components within thecomputing environment, such as a set of software instructions stored ina volatile or nonvolatile of a computer or device having access to thestorage devices (e.g., an operating system process or a hardwaredriver); by a storage system configured to interface with the storagedevices (e.g., a RAID controller); or in respective storage devices ofthe storage set.

As a fourth variation of this first aspect, the volatile memoryrepresentation generated and used by the techniques presented herein maypresent many structures. As one such example, the volatile memoryrepresentation may be structured as a set of interrelated records orobjects, such as a logical disk set comprising logical disksrespectively represented as a sequence of slabs; a slab set comprisingslabs respectively represented as a collection of extents; and an extentset comprising extents respectively represented as a storage device, aphysical address, and an extent length. Moreover, this collection ofinterrelated records or objects may represent a particular organization,e.g., a hierarchical structure comprising a first hierarchical levelrepresenting the logical disks; a second hierarchical level representingthe slabs; and a third hierarchical level representing the extents.Locations within a logical disk manifested by the storage device set maytherefore be located by first identifying the slab comprising thelocation of the logical disk, and then identifying at least one extentwithin the slab comprising the location.

As a fifth variation of this first aspect, the accessing of the storagedevice set through the techniques presented herein may enable particulartypes of flexibility in the storage device set. As a first example, afirst slab may be allocated for a logical disk specifying a first layoutplan, and a second slab allocated for the logical disk specifying asecond layout plan that is different from the first layout plan. As asecond example, at least one capacity request received from anapplication specifying a slab usage, and the provisioning componentconfigured to select a layout plan according to the slab usage of theslab. In particular, a first slab allocated for a logical disk having afirst slab capacity, and a second slab allocated for the logical diskhaving a second slab capacity that is different from the first slabcapacity.

As a sixth variation of this first aspect, many types of layout plansmay be represented by the storage set. An exemplary layout plan set mayinclude, e.g., a monolithic layout plan specifying an allocation ofextents on one storage device; a striping layout plan specifying anallocation of extents on at least two storage devices that togetherprovide the capacity of the slab allocation request; a mirroring layoutplan specifying an allocation of extents that respectively provide thecapacity of the slab allocation request and that are mirrored; and averifying layout plan specifying an allocation of at least one dataextent on at least one storage device that together provide the capacityof the slab allocation request, and at least one verifier extent on atleast one other storage device that stores verifiers for the dataextents. Those of ordinary skill in the art may devise many variationsin the scenarios, organizations, and uses of the techniques presentedherein.

D2. Variations of Techniques

A second aspect that may vary among embodiments involves the applicationof the techniques presented herein.

As a first example, upon receiving the allocation request to generatethe storage region, an embodiment may refrain from initializing thestorage region. As a second example, where at least one location setcomprises at least two data locations respectively storing a data value,the parity location storing a parity value of the data values of thedata locations. In such scenarios, upon receiving a write request towrite at least one data value to respective locations of a location set,the embodiment may write the data values to the data locations;calculate a parity value for the location set; store the parity value atthe parity location for the location set; and update the usage bitmap toindicate values existing at the locations of the location set. Anembodiment may also calculate the parity value by, if the usage bitmapindicates data values existing at the location set, reading stored datavalues that are stored at other data locations of the location set;calculating the parity value using the stored data values and the datavalues of the write request; and if the usage bitmap does not indicatevalues existing at the location set, calculating the parity value usingthe data values of the write request and default values for other datalocations of the location set. Moreover, where the write comprises atleast one unwritten location of the location set not having a datavalue, the embodiment may, if the usage bitmap does not indicate valuesexisting at the location set, write default values at respectiveunwritten locations of the location set. Alternatively, where the writerequest specifying data values for all data locations of the locationset; the embodiment may calculate the parity value using the data valuesof the write request.

As another example, an embodiment may, upon receiving a request todiscard the values of a location set, update the usage bitmap toindicate that values do not exist at the locations of the location set.

As another example, where respective location sets comprising at leasttwo resilient values, an embodiment may, upon detecting a failure of alocation of the location set, determine whether the usage bitmapindicates that values exist for the location set. If so, the embodimentmay use other resilient values of the location set, restore the value atthe location set. However, if not, the embodiment may refraining fromrestoring the value at the location set.

As another example, an embodiment may store the usage bitmap in thestorage set. In particular, for storage sets comprising a journalconfigured to stage write requests to locations of the storage set, anembodiment may updating the usage bitmap comprising: updating the usagebitmap upon destaging a write request to a location of the storage set.For example, the journal of the storage set may comprise a log with ahead pointer and a tail pointer, and writes to the storage set may firstbe stored in the journal (e.g., between the head pointer and the tailpointer), until they are eventually written out to the storage region.In such embodiments, rather than frequently writing the usage bitmap tothe storage device, an embodiment may write the usage bitmap on thestorage device just before writes are evicted from the journal (e.g., byincrementing the tail pointer past the values in the journal). Thisalternative may be feasible, e.g., because if the storage device failsprior to evicting the values from the journal, the written values arestill stored in the journal on the physical medium, and the device mayrecover from the failure by replaying the writes in the journal. Asanother example, the storage set may comprise at least two storageregions and a usage bitmap array comprising usage bitmaps respectivelystoring indications of values existing in respective locations of astorage region.

As another example, the usage bitmap may be stored by compressing theusage bitmap to generate a compressed usage bitmap, and storing thecompressed usage bitmap on the storage device. Moreover, it may beadvantageous to compress the usage bitmap by run-length encoding theusage bitmap. Moreover, the embodiment may compute a compression ratioof the compressed usage bitmap, and may store the compressed usagebitmap in the storage set if the compression ratio is below acompression ratio threshold, but to store the uncompressed usage bitmapin the storage set if the compression ratio is not below the compressionratio threshold. As another example, an embodiment may be configured tostore at least two usage bitmaps in at least two locations of thestorage set, and to, upon receiving a usage bitmap read request to readthe usage bitmap, read a latest usage bitmap in the usage bitmapsequence. For example, respective usage bitmaps stored with a sequencenumber, and each successive storage of the storage bitmap may be storedwith a higher sequence number than the other usage bitmaps stored in thestorage set (e.g., a continuously incrementing logical sequence number).Alternatively, the storage set may be stored in a usage bitmap logcomprising at least two usage bitmap slots and a latest usage bitmappointer. The usage bitmap may therefore be stored by incrementing thelatest usage bitmap pointer, an storing the usage bitmap at the usagebitmap slot indicated by the latest usage bitmap pointer, and read bythe usage bitmap in the usage bitmap slot indicated by the latest usagebitmap pointer. Those of ordinary skill in the art may devise manyvariations

E. Computing Environment

FIG. 3 and the following discussion provide a brief, general descriptionof a suitable computing environment to implement embodiments of one ormore of the provisions set forth herein. The operating environment ofFIG. 3 is only one example of a suitable operating environment and isnot intended to suggest any limitation as to the scope of use orfunctionality of the operating environment. Example computing devicesinclude, but are not limited to, personal computers, server computers,hand-held or laptop devices, mobile devices (such as mobile phones,Personal Digital Assistants (PDAs), media players, and the like),multiprocessor systems, consumer electronics, mini computers, mainframecomputers, distributed computing environments that include any of theabove systems or devices, and the like.

FIG. 3 illustrates an example of a system 500 comprising a computingdevice 302 configured to implement one or more embodiments providedherein. In one configuration, computing device 302 includes at least oneprocessing unit 306 and memory 308. Depending on the exact configurationand type of computing device, memory 308 may be volatile (such as RAM,for example), non-volatile (such as ROM, flash memory, etc., forexample) or some combination of the two. This configuration isillustrated in FIG. 3 by dashed line 304.

In other embodiments, device 302 may include additional features and/orfunctionality. For example, device 302 may also include additionalstorage (e.g., removable and/or non-removable) including, but notlimited to, magnetic storage, optical storage, and the like. Suchadditional storage is illustrated in FIG. 3 by storage 310. In oneembodiment, computer readable instructions to implement one or moreembodiments provided herein may be in storage 310. Storage 310 may alsostore other computer readable instructions to implement an operatingsystem, an application program, and the like. Computer readableinstructions may be loaded in memory 308 for execution by processingunit 306, for example.

The term “computer readable media” as used herein includes computerstorage media. Computer storage media includes volatile and nonvolatile,removable and non-removable media implemented in any method ortechnology for storage of information such as computer readableinstructions or other data. Memory 308 and storage 310 are examples ofcomputer storage media. Computer storage media includes, but is notlimited to, RAM, ROM, EEPROM, flash memory or other memory technology,CD-ROM, Digital Versatile Disks (DVDs) or other optical storage,magnetic cassettes, magnetic tape, magnetic disk storage or othermagnetic storage devices, or any other medium which can be used to storethe desired information and which can be accessed by device 302. Anysuch computer storage media may be part of device 302.

Device 302 may also include communication connection(s) 316 that allowsdevice 302 to communicate with other devices. Communicationconnection(s) 316 may include, but is not limited to, a modem, a NetworkInterface Card (NIC), an integrated network interface, a radio frequencytransmitter/receiver, an infrared port, a USB connection, or otherinterfaces for connecting computing device 302 to other computingdevices. Communication connection(s) 316 may include a wired connectionor a wireless connection. Communication connection(s) 316 may transmitand/or receive communication media.

The term “computer readable media” may include communication media.Communication media typically embodies computer readable instructions orother data in a “modulated data signal” such as a carrier wave or othertransport mechanism and includes any information delivery media. Theterm “modulated data signal” may include a signal that has one or moreof its characteristics set or changed in such a manner as to encodeinformation in the signal.

Device 302 may include input device(s) 314 such as keyboard, mouse, pen,voice input device, touch input device, infrared cameras, video inputdevices, and/or any other input device. Output device(s) 312 such as oneor more displays, speakers, printers, and/or any other output device mayalso be included in device 302. Input device(s) 314 and output device(s)312 may be connected to device 302 via a wired connection, wirelessconnection, or any combination thereof. In one embodiment, an inputdevice or an output device from another computing device may be used asinput device(s) 314 or output device(s) 312 for computing device 302.

Components of computing device 302 may be connected by variousinterconnects, such as a bus. Such interconnects may include aPeripheral Component Interconnect (PCI), such as PCI Express, aUniversal Serial Bus (USB), firewire (IEEE 1394), an optical busstructure, and the like. In another embodiment, components of computingdevice 302 may be interconnected by a network. For example, memory 308may be comprised of multiple physical memory units located in differentphysical locations interconnected by a network.

Those skilled in the art will realize that storage devices utilized tostore computer readable instructions may be distributed across anetwork. For example, a computing device 320 accessible via network 318may store computer readable instructions to implement one or moreembodiments provided herein. Computing device 302 may access computingdevice 320 and download a part or all of the computer readableinstructions for execution. Alternatively, computing device 302 maydownload pieces of the computer readable instructions, as needed, orsome instructions may be executed at computing device 302 and some atcomputing device 320.

F. Usage of Terms

As used in this application, the terms “component,” “module,” “system”,“interface”, and the like are generally intended to refer to acomputer-related entity, either hardware, a combination of hardware andsoftware, software, or software in execution. For example, a componentmay be, but is not limited to being, a process running on a processor, aprocessor, an object, an executable, a thread of execution, a program,and/or a computer. By way of illustration, both an application runningon a controller and the controller can be a component. One or morecomponents may reside within a process and/or thread of execution and acomponent may be localized on one computer and/or distributed betweentwo or more computers.

Furthermore, the claimed subject matter may be implemented as a method,apparatus, or article of manufacture using standard programming and/orengineering techniques to produce software, firmware, hardware, or anycombination thereof to control a computer to implement the disclosedsubject matter. The term “article of manufacture” as used herein isintended to encompass a computer program accessible from anycomputer-readable device, carrier, or media. Of course, those skilled inthe art will recognize many modifications may be made to thisconfiguration without departing from the scope or spirit of the claimedsubject matter.

Various operations of embodiments are provided herein. In oneembodiment, one or more of the operations described may constitutecomputer readable instructions stored on one or more computer readablemedia, which if executed by a computing device, will cause the computingdevice to perform the operations described. The order in which some orall of the operations are described should not be construed as to implythat these operations are necessarily order dependent. Alternativeordering will be appreciated by one skilled in the art having thebenefit of this description. Further, it will be understood that not alloperations are necessarily present in each embodiment provided herein.

Moreover, the word “exemplary” is used herein to mean serving as anexample, instance, or illustration. Any aspect or design describedherein as “exemplary” is not necessarily to be construed as advantageousover other aspects or designs. Rather, use of the word exemplary isintended to present concepts in a concrete fashion. As used in thisapplication, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or”. That is, unless specified otherwise, or clearfrom context, “X employs A or B” is intended to mean any of the naturalinclusive permutations. That is, if X employs A; X employs B; or Xemploys both A and B, then “X employs A or B” is satisfied under any ofthe foregoing instances. In addition, the articles “a” and “an” as usedin this application and the appended claims may generally be construedto mean “one or more” unless specified otherwise or clear from contextto be directed to a singular form.

Also, although the disclosure has been shown and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others skilled in the art based upon a reading andunderstanding of this specification and the annexed drawings. Thedisclosure includes all such modifications and alterations and islimited only by the scope of the following claims. In particular regardto the various functions performed by the above described components(e.g., elements, resources, etc.), the terms used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary implementations of thedisclosure. In addition, while a particular feature of the disclosuremay have been disclosed with respect to only one of severalimplementations, such feature may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular application. Furthermore, to the extent thatthe terms “includes”, “having”, “has”, “with”, or variants thereof areused in either the detailed description or the claims, such terms areintended to be inclusive in a manner similar to the term “comprising.”

What is claimed is:
 1. A method of presenting values of a storage setcomprising: at least one storage region respectively stored at alocation in a location set; a journal configured to store values to bewritten to the storage set; and a usage bitmap, the method performed ona computer having a processor and comprising: executing on the processorinstructions configured to: upon receiving a read request for a value ata location: if the usage bitmap indicates that values exist for thelocation set comprising the location, return the value stored at thelocation; and if the usage bitmap indicates that values do not exist forthe location set comprising the location, return a default value; andbefore evicting at least one value from the journal, store the usagebitmap in the storage set.
 2. The method of claim 1 the storage setcomprising: at least two storage regions, and a usage bitmap arraycomprising usage bitmaps respectively storing indications of valuesexisting in respective locations of a storage region.
 3. The method ofclaim 1, the instructions configured to, upon receiving an allocationrequest to generate the storage region, refrain from initializing thestorage region.
 4. The method of claim 1, at least one location setcomprising: at least two data locations respectively storing a datavalue, and at least one parity location respectively storing a parityvalue of the data values of the data locations.
 5. The method of claim4, the instructions configured to, upon receiving a write request towrite at least one data value to respective locations of a location set:write the data values to the data locations; calculate a parity valuefor the location set; store the parity value at the parity location forthe location set; and update the usage bitmap to indicate valuesexisting at the locations of the location set.
 6. The method of claim 5,calculating the parity value comprising: if the usage bitmap indicatesdata values existing at locations of the location set: reading storeddata values that are stored at other data locations of the location set,and calculating the parity value using the stored data values and thedata values of the write request; and if the usage bitmap does notindicate values existing at the location set, calculating the parityvalue using the data values of the write request and default values forother data locations of the location set.
 7. The method of claim 5: atleast one unwritten location of the location set not having a data valuespecified in the write request; and the instructions configured to, ifthe usage bitmap does not indicate values existing at the location set,write default values at respective unwritten locations of the locationset.
 8. The method of claim 5: the write request specifying data valuesfor all data locations of the location set; and calculating the parityvalue comprising: calculating the parity value using the data values ofthe write request.
 9. The method of claim 1, the instructions configuredto, upon receiving a request to discard the values of a location set,update the usage bitmap to indicate that values do not exist at thelocations of the location set.
 10. The method of claim 1: respectivelocation sets comprising at least two resilient values; and theinstructions configured to, upon detecting a failure of a location ofthe location set: if the usage bitmap indicates that values exist forthe location set, using other resilient values of the location set,restore the value at the location set; and if the usage bitmap indicatesthat values do not exist for the location set, refrain from restoringthe value at the location set.
 11. The method of claim 1, storing theusage bitmap comprising: compressing the usage bitmap to generate acompressed usage bitmap, and storing the compressed usage bitmap on thestorage device.
 12. The method of claim 11, compressing the usage bitmapcomprising: run-length encoding the usage bitmap.
 13. The method ofclaim 11, storing the usage bitmap comprising: computing a compressionratio of the compressed usage bitmap; if the compression ratio is belowa compression ratio threshold, store the compressed usage bitmap in thestorage set; and if the compression ratio is not below the compressionratio threshold, store the usage bitmap in the storage set.
 14. Themethod of claim 11: storing the usage bitmap in the storage setcomprising: storing a usage bitmap sequence of at least two usagebitmaps in at least two locations of the storage set; and theinstructions configured to, upon receiving a usage bitmap read requestto read the usage bitmap, read a latest usage bitmap in the usage bitmapsequence.
 15. The method of claim 14: respective usage bitmaps storedwith a sequence number; and storing the usage bitmap comprising: storingthe usage bitmap with a higher sequence number than the other usagebitmaps stored in the storage set.
 16. The method of claim 14: thestorage set comprising a usage bitmap log comprising: at least two usagebitmap slots, and a latest usage bitmap pointer; storing the usagebitmap comprising: incrementing the latest usage bitmap pointer, andstoring the usage bitmap at the usage bitmap slot indicated by thelatest usage bitmap pointer; and reading the latest usage bitmapcomprising: reading the usage bitmap in the usage bitmap slot indicatedby the latest usage bitmap pointer.
 17. A system for presenting valuesstored in location sets comprising at least two locations of a storageset, the system comprising: a usage bitmap generating componentconfigured to, upon receiving an allocation request to generate astorage region in the storage set, generate in the storage set a usagebitmap indicating, for respective location sets of the storage region,whether values exist in the locations of the location set; and a storageset read component configured to, upon receiving a read request for avalue at a location: if the usage bitmap indicates that values exist forthe location set comprising the location, return the value stored at thelocation; and if the usage bitmap indicates that values do not exist forthe location set comprising the location, return a default value; and ajournal updating component configured to, before evicting at least onevalue from the journal, store the usage bitmap in the storage set. 18.The system of claim 17, at least one location set comprising: at leasttwo data locations respectively storing a data value, and at least oneparity location respectively storing a parity value of the data valuesof the data locations.
 19. The system of claim 18, further comprising: aparity value writing component configured to, upon receiving a writerequest to write at least one data value to respective locations of alocation set: write the data values to the data locations; calculate aparity value for the location set; store the parity value at the paritylocation for the location set; and update the usage bitmap to indicatevalues existing at the locations of the location set.
 20. Acomputer-readable storage medium comprising instructions that, whenexecuted on a processor of a computer having access to a storage device,cause the storage device to present values stored in location setscomprising at least two locations of a storage set of a storage regioncomprising a usage bitmap and a usage bitmap log comprising slots and alatest usage bitmap pointer by: upon receiving a read request for avalue at a location: if the usage bitmap indicates that values exist forthe location set comprising the location, returning the value stored atthe location; if the usage bitmap indicates that values do not exist forthe location set comprising the location, return a default value; uponreceiving the allocation request to generate the storage region, refrainfrom initializing the storage region; upon receiving a write request towrite at least one data value to respective locations of a location set:writing the data values to the data locations by: if the data locationsinclude at least one unwritten location of the location set not having adata value specified in the write request;  writing default values atrespective unwritten locations of the location set; calculating a parityvalue for the location set by: if the write request specifying datavalues for all data locations of the location set, calculating theparity value using the data values of the write request; if the usagebitmap indicates data values existing at the location set: readingstored data values that are stored at other data locations of thelocation set, and calculating the parity value using the stored datavalues and the data values of the write request; and if the usage bitmapdoes not indicate values existing at the location set, calculating theparity value using the data values of the write request and defaultvalues for other data locations of the location set; storing the parityvalue at the parity location for the location set; and updating theusage bitmap to indicate values existing at the locations of thelocation set; upon receiving a request to discard the values of alocation set, update the usage bitmap to indicate that values do notexist at the locations of the location set; upon detecting a failure ofa location of the location set comprising resilient values: if the usagebitmap indicates that values exist for the location set, using otherresilient values of the location set, restore the value at the locationset; and if the usage bitmap indicates that values do not exist for thelocation set, refraining from restoring the value at the location set;store the usage bitmap in the storage set by: compressing the usagebitmap using a run-length encoding algorithm to generate a compressedusage bitmap; computing a compression ratio of the compressed usagebitmap; incrementing the latest usage bitmap pointer, and storing theusage bitmap at the usage bitmap slot indicated by if the compressionratio is below a compression ratio threshold, storing the compressedusage bitmap in the storage set; if the compression ratio is not belowthe compression ratio threshold, storing the usage bitmap in the storageset; upon receiving a usage bitmap read request to read the usagebitmap, reading the latest usage bitmap comprising: reading the usagebitmap in the usage bitmap slot indicated by the latest usage bitmappointer.